1. Technical Field
The description relates to the processing of radar signals.
Various embodiments may refer to the processing of radar signals in the context of road-safety systems.
Various embodiments may refer to radar systems for new-generation motor vehicles.
2. Discussion of the Related Art
A growing number of currently produced vehicles are equipped with radar sensors that may be used, for example, as parking sensors (for warning of the closeness of obstacles and possibly for enabling automatic execution of the parking maneuver) and/or with anti-collision function (for example, to warn the driver that he is approaching too closely another motor vehicle in front of him with the consequent increase in the risk of bumping into said vehicle in the event of sharp braking on the part of said vehicle).
For new-generation vehicles, the extension of the radar functions may be envisaged according to the modalities represented schematically in FIG. 1, namely, in conditions in which a radar system R mounted on board a vehicle V is able to detect, in addition to the presence and to the distance or range of possible obstacles, also further parameters such as, for example, the angular position (azimuth), the speed, and other kinematic parameters of the obstacle (more commonly referred to, in the technical literature on radar systems, as “target”).
Said functions may prove particularly useful in a context such as the one represented schematically in FIG. 1, hence in a situation where the radar R can perform the function of locating, for example, other moving vehicles or pedestrians crossing over a zebra crossing, said vehicles and/or pedestrians constituting, in fact, the “targets” of the action of detection performed by the radar and certainly not the targets for the vehicle V on which the radar R is installed.
Various considerations, for example of reliability of operation, lead to suggesting the use, for a function such as the one described, of LFMCW (Linear Frequency-Modulated Continuous-Wave) radar systems.
A radar of this nature is suited to the implementation of beam-forming/beam-steering functions, with consequent possibility of controlling the conformation of the beam being received and also of controlling steering thereof, i.e., the receiving direction, with the consequent possibility of improving the action of detection of the angular position of the obstacle or target.
As represented schematically in FIG. 2, a radar system of this nature may basically include a radio-frequency section 10 coming under (at least) one transmitting antenna Tx and at least one receiving antenna (usually a plurality of receiving antennas) Rx. Co-operating with the radio-frequency section 10 is a base-band device 12 that is able to perform functions such as acquisition, beam forming/steering, tracking, etc. The aim of this is to yield at output (for example, for visual and/or acoustic presentation on a display provided on the vehicle V equipped with the radar system R) data representing output information OI regarding the range, angular position (azimuth), speed, possibly acceleration, etc., of the targets.
As represented schematically in FIG. 3, the signal TS transmitted by an LFMCW radar is a sequence of chirps centred around a given frequency. For example, for applications in the automotive sector, said frequency may be chosen at a value of 77 GHz. Each chirp may be a frequency-modulated sinusoidal wave with a frequency f varying according to a ramp as a function of time, with a modulation depth BW that may range from some megahertz to some hundreds of megahertz.
The echo signal RS received reflected by the target is substantially a copy (attenuated in amplitude) of the transmitted signal TS delayed in time by an interval Tau corresponding to the round-trip time with respect to the target. The received signal RS may be mixed, according to a general heterodyne receiver scheme, with a (local) replica of the transmitted signal, so as to generate an intermediate-frequency signal IF. After amplification and treatments of various nature (band limiting, filtering) the intermediate-frequency signal IF may be subjected to processing (for example, after analog-to-digital conversion) within the base-band device 12 that generates the information OI on the target.
The diagram of FIG. 4, where Tchirp indicates the duration of each chirp of the signal, represents the main parameters that distinguish the transmitted signal TS and the received signal RS in a LFMCW radar system.
To a fair approximation, the delay Tau and the distance (range) of the target are linked according to the relationTau=2R/c 
where c is the speed of light.
The delay Tau is in turn linked to the frequency IF of the intermediate-frequency signal deriving from mixing according to the relationIF=(BW/Tchirp)·Tau
From the measurement of the frequency IF it is thus possible to arrive at the range.
The information content of the received signal RS is, however, richer, in so far as the received signal RS brings with it information not only on the range R of the target P but also on its movement parameters (for example, speed, acceleration, etc.). Furthermore, if—as is schematically illustrated in FIG. 5—a number of receiving antennas RX is used (which are to be considered co-located with respect to one another, namely located in one and the same position with distances of separation much shorter than the range R where the target is located), operating with modalities basically amounting to beam-forming/beam-steering techniques, it is possible to obtain also information on the angular position (azimuth) of the target P.
For example, as illustrated in Barrick, D. E., “FM/CW radar signals and digital processing,” NOAA Tech. Report ERL 283-WPL 26, U.S. Dept. of Commerce, Boulder, Colo. 1973, the range and speed information may be extracted using a two-dimensional Fourier transform (for example, 2D FFT), applied to an input matrix composed using the echoes of a number of successive and contiguous chirp signals. The presence of a peak in the output matrix of the 2D FFT processing reveals the presence of a target, and from the corresponding indices it is possible to infer an indication of the values of range and of speed. The aforesaid input matrix may include, in each row, the values regarding a single ramp of the chirp signal, while the subsequent rows refer to the subsequent ramps of the chirp signal. Each column of the matrix represents, instead, a given “sample” in the chirp waveform, as received in successive ramps of the chirp signal.
Furthermore, when there is available a set of received signals RS obtained via a number of receiving antennas RX it is possible to combine the signal coming from different antennas and calculate, by applying a beam-forming algorithm that may be implemented completely in the numeric field, the direction of arrival (DOA) of the echo signal, which enables detection of the angular position (azimuth) of the target with respect to the transmitter.
In the possible mode of use exemplified in FIG. 1, a number of targets, such as for example the pedestrians P1, P2, P3, and P4 crossing over on a zebra crossing can be detected by the radar of a vehicle V that is arriving as located at different ranges and in different angular positions (azimuths), with yet further possible differences also as regards other movement parameters, such as for example the speed of pedestrians crossing over a zebra crossing.
The diagram of FIG. 6 illustrates a possible architecture of implementation of a scheme, as represented in general in FIG. 2, regarding a radar system of the type considered herein, i.e., a radar system R that emits sequences of chirp-modulated signals TS in which the signals RS received via a plurality of receiving antennas RX are mixed with local replicas of the or chirp-modulated transmission signals TS to produce, for each receiving antenna RX, a sequence of detection signals, it being possible to subject the detection signals thus obtained to Fourier-transform processing and beam-forming processing so as to generate values of range R, azimuth θ, and speed for at least one target P1-P4 of the radar system R.
In the diagram of FIG. 6 provided by way of example, it is assumed that the transmitting antenna TX (for simplicity of description reference will be made to a single transmitting antenna, even though it is possible to use an antenna array also in transmission, as illustrated in FIG. 5) is supplied by a generator 100 of the chirp transmission signal TS comprised in the radio-frequency stage 10, while the signal in reception R is assumed as coming from a plurality of k receiving antennas RS, each of which transfers to the radio-frequency stage 10 a respective signal in reception. Once mixed in a corresponding mixer 102 with a replica of the signal in transmission TS, each signal in reception produces a respective intermediate-frequency signal component IF. According to various known solutions, the various intermediate-frequency signals can be subjected to operations of filtering for de-noising, automatic gain control, etc. (implemented, for instance, in modules designated as a whole by 104) and to a subsequent analog-to-digital conversion (modules 106) with a view to transfer thereof to the base-band stage 12 dedicated to processing of the detection signals thus obtained.
According to various solutions, the treatment operations in question may be executed in the radio-frequency stage 10 operating on a number k of channels in parallel as many as are the receiving antennas RX and hence the components of the intermediate-frequency signal IF.
A similar structure on a number of channels organized in parallel (even only in a virtual way, i.e., without an effective separation) can be kept in the base-band stage 12 by providing buffer memories 120 from which the data may be transferred to two-dimensional Fourier-transform (2D FFT) processing modules 122.
From here the “FFT transformed” signals may pass on to a further stage 124, which is entrusted with execution of the beam-forming/beam-steering processing as a function of values of angles of azimuth determined by acting on an input 124a. A module 126 set at the output of the module 124 may perform the function of detecting the peaks of the signals deriving from the 2D FFT treatment after beam-forming/beam-steering processing.
The criteria of execution of the processing functions recalled previously are to be deemed in themselves known in the art (also from the bibliographical reference cited previously) and hence such as not to require a detailed description herein.
In this regard, it will be appreciated that, except for what will otherwise be described in what follows, what has been illustrated previously with reference to FIGS. 1 to 6 applies also to the exemplary embodiments.